The immune response is initiated at the level of professional antigen presenting cells (APC), which include dendritic cells (DC) and macrophages (Mg), that reside in tissues throughout the body. DCs express high levels of cell surface molecules and complementary receptors that interact with T lymphocytes and, therefore, induce potent immune responses. DCs also secrete cytokines, chemokines and proteases which initiate immune responses and culminate in the amplification of both cellular and humoral immunity.
DCs express on their surface major histocompatibility complex (MHC) molecules that bind fragments of antigens. T cells which express T cell receptors (TCR) that recognize such antigen-MHC complexes become activated and initiate the immune cascade. In general, there are two types of MHC molecules, MHC class I and MHC class II molecules. MHC class I molecules present antigen to specific CD8+ T cells and MHC class II molecules present antigen to specific CD4+ T cells.
For effective treatment of many diseases, particularly cancers, vaccines must elicit a potent cytotoxic T lymphocyte (CTL) response, also referred to as a cytotoxic T cell response. Cytotoxic T cells predominantly include CD8+ T cells which recognize antigen in the context of MHC class I. The processing of antigens in the context of MHC class I molecules differs significantly from that of MHC class II molecules. Antigens delivered exogenously to APCs are processed primarily for association with MHC class II molecules. In contrast, due to the intracellular location of MHC class I molecules, antigens delivered endogenously to APCs are processed primarily for association with MHC class I molecules. This is not only true for APCs, as all nucleated cells express MHC class I molecules, and are continuously displaying on their surface endogenously produced antigens in association with MHC class I molecules.
For this reason, cells infected with virus or tumor cells expressing unique proteins can be targeted by CTLs when viral or tumor antigens are displayed as a peptide bound to MHC class I molecules. However, DCs, under specific conditions, have the unique capacity also to allow exogenous antigens access to internal compartments for binding to WIC class I molecules, so that they are presented to T cells via both MHC class I and class II pathways. This process is called cross-priming or cross-presentation.
Accordingly, while antibody-mediated responses have demonstrated impressive protective or therapeutic efficacy for specific diseases when directed against particular secreted or cell surface antigens, the most effective immunotherapy for many diseases appears to require T cell-mediated immune responses, particularly CTL responses. Since effective CTL responses are not limited to extracellular antigens, there exist possibilities for developing antigen-based therapeutic vaccines that are not effective antibody targets. Therefore, new methods for generating CTLs in response to disease-associated antigens have been of great interest, as these cells are thought to be critical for the efficacy of many vaccines in general, and essential to most therapeutic cancer vaccines.
One vaccine approach which has been tested to date employs immunizing with antigenic peptides. This method of immunization bypasses the need for antigen uptake and processing and relies on the ability of the peptide to bind directly to MHC class I molecules already expressed on the surface of the APC. Although this method has clearly shown evidence of CTL induction in patients, the method has several limitations. The antigenic peptide must be pre-established, different peptides are required for individuals with different MHC haplotypes, and peptides are short-lived in vivo.
Another approach which has been tested employs antibody-antigen complexes. Paul et al. (62) showed that antibodies specific for a given antigen could enhance humoral immune responses against the antigen in mice, presumably by delivering the immune complexes to Fc receptors for IgG (FcγR) expressed on APCs: Wernersson and colleagues (63) studied the role of individual FcγRs in the enhancement of immune responses using immune complexes in vivo. Their studies demonstrated that FcγRI is sufficient to mediate enhanced immune responses. However, such immune complexes do not target APCs specifically, as they also bind to Fc receptors on many cells that are not involved in antigen presentation, thereby, decreasing the efficiency of antigen delivery.
Subsequent studies have used antibodies to selectively target antigens to a variety of receptors on APCs, and have demonstrated that such selective delivery is capable of inducing humoral responses (66,67). In addition, it has been shown that immune complexes bound to FcR on DCs are processed and presented in context of MHC class I (64,65). Moreover, many such FcR-targeting approaches are limited because FcR are expressed on many non-APC such as platelets and neutrophils. Ideally, a vaccine that targets APC specifically and is capable of inducing an effective MHC class I-restricted CTL response, as well as an effective MHC class II-restricted TH response could offer improved efficacy in treating certain diseases.
Similarly, mannosylated antigens have been shown to induce humoral immune responses and T cell-mediated immune responses, such as CTL responses. However, mannosylated antigens do not target APC specifically due to the significant abundance of other mannose binding proteins. Furthermore, mannosylated proteins are internalized by immature DCs through macropinocytic mechanisms. Therefore, the mechanisms and nature of immune responses generated by mannosylation of antigens differs greatly from that generated by specific targeting of antigens to mannose receptors using antibodies.
Since current methods do not efficiently and specifically target APCs, many therapeutic vaccines require the purification of DC from patients, which are reinfused after exposure to the antigen.
Accordingly, the need exists for improved vaccines capable of efficiently targeting APCs and generating antigen-specific T cell-mediated immune responses, including antigen-specific CTL responses, required for effective treatment of many diseases.